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Subject: Re: [PATCH v2 2/3] KVM: x86: Move pkru save/restore to x86.c
To:     Jim Mattson <jmattson@google.com>, Babu Moger <babu.moger@amd.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Wanpeng Li <wanpengli@tencent.com>,
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        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        mchehab+samsung@kernel.org, changbin.du@intel.com,
        Nadav Amit <namit@vmware.com>,
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <90657d4b-cb2b-0678-fd9c-a281bb85fadf@redhat.com>
Date:   Sat, 9 May 2020 14:59:17 +0200
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On 09/05/20 00:09, Jim Mattson wrote:
>> +       if (static_cpu_has(X86_FEATURE_PKU) &&
>> +           kvm_read_cr4_bits(vcpu, X86_CR4_PKE) &&
>> +           vcpu->arch.pkru != vcpu->arch.host_pkru)
>> +               __write_pkru(vcpu->arch.pkru);
> This doesn't seem quite right to me. Though rdpkru and wrpkru are
> contingent upon CR4.PKE, the PKRU resource isn't. It can be read with
> XSAVE and written with XRSTOR. So, if we don't set the guest PKRU
> value here, the guest can read the host value, which seems dodgy at
> best.
> 
> Perhaps the second conjunct should be: (kvm_read_cr4_bits(vcpu,
> X86_CR4_PKE) || (vcpu->arch.xcr0 & XFEATURE_MASK_PKRU)).

You're right.  The bug was preexistent, but we should fix it in 5.7 and
stable as well.

>>  }
>>  EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
>>
>>  void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
>>  {
>> +       /*
>> +        * eager fpu is enabled if PKEY is supported and CR4 is switched
>> +        * back on host, so it is safe to read guest PKRU from current
>> +        * XSAVE.
>> +        */
> I don't understand the relevance of this comment to the code below.
> 

It's probably stale.

Paolo